It is often necessary or desirable to run tools or sensors into drilled bores, such as are utilized in the oil and gas industry to access subsurface hydrocarbon-bearing formations. These operations often involve use of spoolable support members, such as wireline. In comparison to operations in which tools are mounted on jointed pipe, for example drill pipe, such operations allow an operator to lower and raise tools and sensors in the bore at a relatively rapid rate, using relatively straightforward surface apparatus.
An example of the surface equipment used in wireline operations is illustrated in FIG. 1 of the accompanying drawings. This equipment comprises three major component parts: a wireline unit, of which the principal component is a wireline winch; the wire itself, and a lubricator assembly.
The wireline unit includes a winch drum around which the wireline is reeled. The winch drum is driven by a prime mover, for example an electric motor. The winch unit incorporates a mechanical measuring device, or “Depth Counter”, which measures the length of wireline that has been deployed and thus is used to indicate to the operator the distance tools have travelled up or down the hole. The device utilizes a measuring wheel around which the wire is wrapped, and the revolutions of this wheel are correlated to depth and displayed on a recorder or depth gauge. A weight/tension indicator may be connected to the hay pulley that guides the wire from the wireline unit up to the top of the lubricator assembly. In more sophisticated wireline units, a two or three wheel wraparound depth measuring device may be provided with an integral load cell for wire tension measurement.
The depth measurement and wire tension are perhaps the two key operating parameters during a well service operation. The winch operator will run a wireline-mounted tool into the well to a predetermined desired depth using the depth measuring device while changes in wire tension are monitored and may be used to determine downhole tool engagement and disengagement. The operator may also use the wire tension indicator to ensure that the wire is not subject to a tension likely to damage the wire.
The wireline form may vary widely, being selected to suit the particular nature of the wireline work. The wireline may be several kilometers long and formed of special alloy, and is thus likely to represent a significant investment on the part of the wireline provider/operator.
The lubricator assembly is a tubular pressure chamber arrangement which may be fixed to the top of the wellhead and permits tools and other equipment to be placed into the well and subsequently retrieved. In operation, the lubricator is somewhat similar to an air lock, in that it is operable to isolate the interior of the lubricator from well pressure, such that the lubricator may then be opened and tools and sensors secured to the end of the wireline. However, when tools are downhole, the lubricator is subjected to full wellhead pressure.
The lubricator is assembled in sections and connected with ‘quick unions’ having integral pressure seals. There must also be a seal around the wire at the top of the lubricator to prevent well pressure escaping from the entry point into the lubricator. The seal must be efficient enough to prevent pressure escape, but also allow movement of the wire. A wireline stuffing box performs this function for single strand wireline such as slick or piano wire.
The wireline stuffing box normally serves two functions. In combination with a sheave wheel mounted on the upper end of the lubricator assembly, the stuffing box guides the wire from the bottom hay pulley into the top of the lubricator assembly. The stuffing box also contains well pressure while the wireline is moving or standing still. Sealing around the wire within the stuffing box is achieved using mechanical or hydraulically energized stacks of packing elements.
Braided wire requires a more sophisticated system of pressure sealing than single strand wire. In one arrangement highly viscous oil is pumped continuously into the lubricator and into flow tubes through which the braided line passes. The oil fills the small annular space between the wire and the inside of the flow tube to prevent well pressure escaping past the wire. The oil exits at the top of the lubricator and is channeled into drums for disposal.
The lower end of the lubricator assembly includes a wireline blowout preventer (BOP) having moveable rams with shaped rubber elements which may close and seal on the wire without damaging the wire. This allows wellbore pressure to be contained below the BOP, allowing depressurization above and opening of the lubricator.